In many industrial processes, the presence and amount of even minute moisture concentrations in flowing gas streams must be detected and measured with a high degree of speed and accuracy. The process of manufacturing semiconductors uses flowing gas streams, for example, and any moisture present in those streams affects production yield. If moisture concentrations exceed specified limits, the contaminated gas stream may produce, at considerable expense, an unacceptable semiconductor lot. Thus, detection and measurement of moisture concentrations in industrial processes such as semiconductor production is required because moisture is often critical to the quality of the product produced.
To meet the industrial demand, sensitive hygrometers are available which have extremely low detection limits and fast response times. The most sensitive and commercially available hygrometers can detect and measure moisture concentrations on the order of ten parts per billion by volume--although modern, high-purity hygrometers may reach limits of a few ppb. K. Sugiyama & T. Ohmi, "Ultraclean Gas Delivery Systems--Part I", in Microcontamination at 49-54 (Nov. 1988), discloses that gases with moisture levels on the order of two parts per billion can be produced and that such levels can be measured by Atmospheric Pressure Ionization Mass Spectrometry (APIMS). See also T. Kimura, J. Mettes & M. Schack, "Sub-ppb Analysis of Nitrogen Gas by APIMS", presented at the Technical Symposium of SEMICON EAST 89 in Boston, Mass. (Sept. 1989) (disclosing an experimental setup and a procedure for the analysis of high-purity nitrogen). The commercial devices usually include an alarm, which is activated once the moisture concentration of a sample gas stream exceeds a specified level.
Modern hygrometers also typically incorporate an electrolytic cell as the analytical component. The gas to be measured flows through the cell with a known flow rate. The moisture concentration of the gas is determined by absorbing the moisture from the gas, using an hygroscopic film, and electrolyzing the water absorbed in that film. Once equilibrium is achieved, the number of molecules electrolyzed per second, measured as the electrolysis current, is proportional to the number of water molecules entering the cell with the gas each second. An example of an electrolytic cell is described in U.S. Pat. No. 4,800,000 to D. A. Zatko, incorporated herein by reference.
Conventional hygrometers tend to react slowly to changes in moisture concentration when measuring very small concentrations. An unacceptable time lag of the hygrometer may occur, especially in response to a rise in moisture concentration, after the hygrometer is connected to a very dry gas for a long period. The presence of dry gas for a long time will cause the components of the hygrometer which contact the gas to become dry themselves. Those components include packing materials, like epoxy, which are known to be relatively porous and to absorb or emit moisture from or into a passing gas stream. Such components are described in a co-pending U.S. application Ser. No. 07/629,439 entitled "Counterflow Device and Method to Reduce the Negative Impact of Contaminating Materials Used in Moisture Sensitive Apparatuses or Procedures" and filed on Dec. 18, 1990 by Jacob Mettes. That application is incorporated herein in its entirety.
When an hygrometer encounters a dry gas having a moisture concentration below its detection limit, the instrument will produce a background level reading. In contrast to that reading and in reality, however, the hygrometer and its components will attain an equilibrium corresponding to the lower (undetectable) moisture level. When the moisture concentration subsequently changes to a higher level, certain internal components of the hygrometer will, because they are dry, absorb the moisture before the gas reaches the analyzer. Consequently, it will be some time before the hygrometer senses the increased moisture and can activate an alarm or show the higher concentration.
The amount of time depends, among other things, on how dry the gas was and on how long the dry gas flowed. The process monitored by the hygrometer may be using gas with an unacceptably high moisture concentration for a relatively long time, therefore, before the hygrometer "reads" the correct concentration and activates an alarm. For many applications, such a time lag is unacceptable.
In addition to the time lag discussed above, conventional hygrometers fail to address another matter: they do not provide automatic verification that the system is functioning. Many users of hygrometers would like verification that their instrument remains responsive to moisture. Users now verify responsiveness on occasion by introducing air and, therefore, moisture into the hygrometer and monitoring the hygrometer's response.
Although verification is achieved by that approach, there is little quantitative control on the amount of moisture introduced. The amount is, in fact, usually large. Consequently, the hygrometer components which absorb moisture will become wet at moisture concentrations typically far above the alarm levels. In such a case, the absorbed moisture will desorb slowly back into the gas flow once resumed and it will be some time before the hygrometer will effectively read the low moisture concentrations in the gas. Moreover, the approach is typically not automatic; the user must consciously decide to test the hygrometer.